1. Field of the Invention
The present invention relates to a color image display apparatus which displays a color video image by controlling light emission of red (R), green (G) and blue (B) primary colors, and more particularly, to a color image display apparatus with an excellent dynamic resolution characteristic, which displays a high-quality moving image where color fringes at moving image edges are inconspicuous.
2. Description of the Prior Art
In recent years, in place of conventional Braun tube (CRT) display devices, flat-panel type display devices are becoming popular. These thin and light display panel devices, having a display panel where liquid crystal or plasma is sealed, display images with reduced image distortion, and receives reduced influence of earth magnetism. Among the flat-panel display devices, a plasma display device particularly draws public attention as a next-generation color image display device. The plasma display device is spontaneous light emitting device, and therefore it has a wide view angle. Further a large panel can be relatively easily constructed for this device. In this flat-panel display device, one pixel consists of red (R), green (G) and blue (B) light emitting cells. Color image display is realized by controlling the light emitting luminance levels of the respective light emitting cells.
Further, the plasma display device or the like having difficulty in displaying gray scale representation between "light emission (turned on)" and "non light emission (turned off)", employs a so-called subfield method for displaying the gray scale representation by controlling the light emitting luminance levels of the respective R, G and B light emitting cells. In the subfield method, one field is divided into a plurality of subfields on a time base, then light emitting weights are uniquely allotted to the respective subfields, and light emission in the respective subfields are on/off controlled. This attains luminance gradation (or tonality) representation.
For example, in a case where one field is divided into six subfields SF0 to SF5 and light emitting weights in the ratios 1:2:4:8:16:32 are respectively allotted to the subfields, 64 level gradation can be represented. At level "0", light emission is not performed in any of the subfields SF0 to SF5. At level "63" (=1+2+4+8+16+32), light emission is performed in all the six subfields.
In this manner, in the color image display device which controls the light emitting luminance levels of respective R, G and B light emitting cells by the subfield method, the image quality of a displayed moving image is greatly influenced by time response characteristics related to light emission by the R, G and B cells (hereinafter may be simply referred to "light emitting response characteristics") and the array of light emitting weights allotted to the respective subfields in each field.
The light emitting response characteristics of the R, G and B cells respectively indicate a light-emitting rise time characteristic from a point where a controller has instructed to start light emission to a point where light emitting luminance at the cell actually reaches a desired level, and a persistence time characteristic after the light emission instruction. Generally, if the persistence time is long, the light-emitting rise time is long. Accordingly, the persistence time is used as a representative characteristic of light emitting response characteristic. In the following description, the light emitting response characteristic is represented by the "persistence time" (a period from a point where the light emission is at the peak to a point where the light emission is at a level 1/10 of the peak). The "persistence time" includes the "light-emitting rise time characteristic".
The operation of this color image display device can be ideal operation as the light emitting response characteristics are short, however, the light emitting response characteristics cannot be reduced to zero. Further, as the light emitting response characteristics greatly depend on physical characteristics such as fluorescent materials used as the light emitting cells, it is very difficult to obtain uniform response characteristics in the R, G and B cells having different luminous wavelengths. For these reasons, when a moving image is displayed, the differences in time responses of the respective light emitting cells cause time lags in R, G and B light emission which overlap with each other, resulting in color shift (color fringing). The color shift appears at an edge portion where luminance greatly changes, e.g., from black to white or vice versa, as a phenomenon that a color different from the original image color is perceived. This seriously degrades image quality in moving image display.
Hereinbelow, the process of occurrence of color fringing interference at edge portions will be described with reference to FIG. 3 and FIGS. 4A and 4B. As shown in FIG. 3, a white rectangular pattern 32 on black background 31 is displayed on a display screen of a display device, and the white rectangular pattern 32 is moved rightward in FIG. 3. FIGS. 4A and 4B show color fringes occurred on the boundaries between white and black colors.
FIG. 4A shows the intensities (amplitudes) in the respective light emitting cells. FIG. 4B shows colors displayed on the display screen. As shown in FIG. 4A, as the G light emitting response is slower than the R and B light emitting responses, the G light emitting response represented with the broken line is delayed from the R and B light emitting responses represented with the solid lines. Thus, color fringing occurs in edge areas A and B. As shown in FIG. 4B, in the edge area A, a color of magenta (R+B) is perceived due to shortage of the amplitude of G with respect to R and B. In the edge area B, a color of green (G) is perceived due to excess amplitude of G. The edge area where color fringing occurs becomes wider as the speed of moving image increases.
In this manner, in the white and black video signal, colors not included in the original image (magenta and green) are perceived depending on the motion of the image. This seriously degrades the image quality. Especially, in the plasma display device and the like, material having persistence time of 12 ms or longer is often used as a G light emitting cell. As the response of the G cell using this material is slower than the responses of R and B cells, the consequent color fringing in edge areas is a main factor of degradation of image quality.
On the other hand, in the display devices which displays gray scale representation by the subfield method, the dynamic resolution is greatly influenced by the array of light emitting weights for the respective subfields in each field. To prevent degradation of dynamic resolution, it is preferable to perform light emission, based on a video signal that arrives for one field, as impulses for a very short period within each field period. In the CRT display devices, one field period is required for horizontal and vertical scan processing, however, impulse-like light emission is made for one pixel at a particular display screen position, in each field.
However, in the gradation representation by the subfield method, as the video signal that arrives for one field is divided into a plurality of subfields within the field for light emission and display, impulse light emission cannot be made for a short period. For this reason, it is difficult to realize a dynamic resolution characteristic equivalent to that of the CRT device.
Hereinbelow, the phenomenon where the dynamic resolution is degraded in correspondence with the array of light emitting weights for subfields will be described with reference to FIG. 5, FIGS. 6A and 6B and FIGS. 7A and 7B. In this case, the white rectangular pattern 32 shown in FIG. 3 is displayed by a display device having a subfield arrangement for 64 (level "0" to level "63") level representation with six subfields in FIG. 5. In a white (level "63") pixel, light emission is performed in all the subfields SF0 to SF5 in one field, and the ratios of light emission intensities are 16:4:1:2:8:32. This means the array of light emitting weights is made such that energy concentrates at the head and the end of the field.
FIG. 6 shows a v-shaped angular light-emitting luminance distribution in a case where light emitting weights for the subfields are arranged such that the light emitting weight gradually decreases and then gradually increases in each of field 1, field 2, . . . of sequentially inputted video signals. In this v-shaped light emission type subfield arrangement, light emission most highly concentrates around a boundary T1 between fields, and intense light emission occurs at field periods. In the boundary T1, light emission in the first field and that in the second field mix with each other. When the moving rectangular pattern is displayed, two images overlap with each other with a time lag therebetween as represented with the solid line in FIG. 7A. Thus, an image with seriously degraded resolution is perceived.
For example, if light emitting response time of the G-cell is slow, a pattern represented with the broken line in FIG. 7A is detected. Similar to FIGS. 4A and 4B, in edge areas A1 and A2, a color of magenta is perceived due to shortage of amplitude of G light emission, and in edge areas B1 and B2, a color of green is perceived due to excess amplitude of G light emission.
In this case, as the two images overlap with each other with a time lag therebetween, the resolution is degraded, and the luminance does not change abruptly. Accordingly, in comparison with the color fringing in FIGS. 4A and 4B, the range of interference is wider, while the density of false colors (magenta and green) is lower. In this manner, the arrangement of light emitting weights for the subfields and the response characteristics of the R, G and B cells are closely related with each other. As the arrangement of light emitting weights for the subfields reduces color fringing interference at edge portions due to the differences in light emitting response characteristics of the R, G and B cells, both characteristics must be optimized so as to realize high-quality moving image reproduction.
Note that the gradation representation by using the subfield method is disclosed in Japanese Examined Patent Publication No. 51-32051, for example, and a method to reduce false contour noise characteristic of the subfield method is disclosed in Japanese Examined Patent Publication No. 4-211294, for example.
In the above-described conventional color image display devices, regarding the light emitting response characteristics of R, G and B cells, the image quality of a still image is treated as first priority. In those devices, fluorescent materials are selected in consideration of chromaticity coordinates, white balance conditions and luminous efficiency and the like, however, light emitting response characteristics based on the image quality of a moving image have not been considered, otherwise, even if considered, the light emitting response characteristics of the respective cells are shortened as much as possible only to reduce persistence.
Further, in the subfield method, the array of light emitting weights for subfields is determined only to reduce flicker or false contour interference, characteristic of this method, however, the degradation of dynamic resolution characteristic has not been considered.
Further, in the conventional color image display devices, the interaction between the light emitting response characteristics of R, G and B cells and the array of light emitting weights for subfields has not been considered.
Accordingly, in the above-described conventional color image display devices, when a moving image is displayed, R, G and B light emission timings shift from each other due to the differences in light emitting response characteristics of R, G and B cells. Therefore, a color not included in the original image is perceived at an edge portion, and the image quality is seriously degraded.
Further, even in a case where the light emitting response characteristics of R, G and B cells are increased, if the arrangement of light emitting weights for subfields is inappropriate, the dynamic resolution characteristic cannot be improved.
Generally, when one field is divided into M subfields, and light emitting weights corresponding to powers of 2 are allotted to the subfields, gradation representation can be made to the maximum level 2.sup.M. However, if light emitting weights which are not powers of 2 are allotted to the subfields or the subfields are divided so as to perform processing to remove false contour, characteristic of the subfield method, the number L of display gray scale levels for each pixel, with respect to the number M of the subfields, is less than 2.sup.M. That is, the number of subfields increases to realize the same display gray scale level. In this manner, when the number of subfields has increased, light emission is dispersedly performed within one field, which degrades the dynamic resolution.